The present Invention relates to rotary Impact, torque intensifying apparatus for use with drill bits, particularly polycrystalline diamond compact xe2x80x9cPDCxe2x80x9d bits and methods of use applied to subterranean drilling.
Conventional drill bits include roller bits which use compression to crush rock at the toolface when drilling a wellbore in a subterranean formation. It is known to apply axial impact assemblies for enhancing the compressive breaking action of percussive bits.
PDC bits, however, use a shearing action to break the material of the formation. Excessive axial force on a PDC bit is a known cause of failure of the cutters.
The PDC cutters and inserts of PDC bits are subject to failure through vibration and impact. Ideally, a PDC bit has continuous loading while shearing material at the toolface. However, when the rate of penetration suddenly slows, or when a hard interface is encountered, such as a stringer, the bit slows or hangs up, possibly even temporarily ceasing to rotate. Despite slowing or cessation of rotation of the drill bit, the drill string continues to rotate. Whether the bit is at the end of a rotating drill string, or at the end of a coiled tubing BHA, the rotary drive continues to wind up the drill string, building up torque and potential energy. Typically, the torque reaches a certain elevated level and the bit finally releases and spins violently, either due to the energy built up or due to a shortening of the drill string as it winds up. The sustained release of energy as the bit spins causes chatter or repeated impacts of the PDC cutters against the rock facexe2x80x94causing significant damage to the PDC bit cutters.
It Is an expensive process to trip out and replace a damaged PDC bit.
It is believed that PDC bit failure is caused by the chatter and impact associated with the sustained and violent release of the built up torque. Nevertheless, the lock up of a PDC bit is a known and persistent problem resulting in expensive down time and equipment cost
In a surprising discovery, PDC bit performance is improved and incidences of failure can be reduced by repeatedly applying increased torque at the PDC bit through the use of a rotary impact tool. So as to avoid large build up of torque and to suffer the associated sustained impact damage to a PDC bit on release, an assembly is provided for introducing a consistent series of smaller and localized rotary impacts to the bit, avoiding lockup and potentially damaging energy storage in the drill string.
The present invention implements a method and apparatus for increasing the drilling effectiveness of PDC bits while minimizing failures due to the release of energy following windup.
Simply, the method comprises increasing the effective torque of the drill bit by repeatedly and periodically intensifying the torque at the PDC drill bit. The periodic increases in torque avoid the potential for build-up of torque on bit lockup or sustained high torque incidences which are associated with PDC bit failure when the built-up of torque is released. Preferably, introduction of rotary impact is applied only during drilling.
In an apparatus aspect, a rotary torque impacting assembly is positioned between the drill bit and the rotary drive such as a rotary drill string or a downhole motor. The drill bit is adapted for rotation by the assembly which provides the nominal torque necessary to develop the shear forces used by the PDC bit to cut the formation. An energy source in the impacting assembly supplements the nominal torque provided by the rotary drive. Preferably, a drilling fluid driven turbine in the assembly drives a rotary hammer for periodic impacts with an anvil connected through to the drill bit.
The assembly comprises an output bit shaft for connection to the drill bit, and a housing for connection to the rotary drive. The bit shaft has a lower connection to the bit and an upper shaft end which projects into the downhole end of the housing and is rotatably driven thereby. The upper shaft end is fitted with a rotary anvil. The housing further houses a motor which rotates a hammer about the bit shaft""s anvil. The motor spins the hammer and builds up its potential energy. When the anvil and hammer connect, the potential energy is released into the upper shaft end and thus into the drill bit, increasing its instantaneous torque and hence to cut through the difficult formation. For increased effectiveness, the bit shaft is adapted for permitting limited rotational freedom relative to the driving housing so that the bit shaft receives substantially all of the rotary impact. Preferably, the hammer""s motor is impeded from operation when the bit is off bottom and not drilling.